The invention deals with a method for the automatic control of the locking clutches of the differentials of an all-wheel driven vehicle, the differentials each having an input shaft and two output shafts and, starting from pulses corresponding to the rotation of the two output shafts, in each case slip signals being formed which, as soon as they exceed a threshold value, initiate the driving of the locking clutch of the respective differential.
A device for practicing a method of this type in the case of an all-wheel driven vehicle having a plurality of differentials (in-axle differentials, inter-axle differential) is known from DE 43 27 507 C2. Using the measures described there, it is ensured that in all driving situations the correct locking clutch is actuated at the correct instant. This also requires that locking clutches are not unnecessarily switched, in order to impede steering as little as possible. For this reason, taking into account the steering lock is also provided.
However, in the case of vehicles in which steering at a standstill already leads to considerable rotation of individual wheels, which is perceived by the control system as slip, then locking of a differential may occur inopportunely, which severely impedes the steering and leads to unnecessary stresses in the drive train. This is not an effect of the steering lock, which can otherwise already be taken into account in the threshold value, but an effect of the change in said steering lock, that is to say in its first derivative.
Vehicles of this type are either vehicles with articulated-vehicle steering or vehicles (for example construction vehicles) having steering-knuckle steering, whose axis geometry necessitates a large steering rolling radius. The latter fact means that the extension of the steering axis intersects the ground at a relatively great distance from the point on which the wheel stands. In the case of vehicles with articulated-vehicle steering, the articulation axis does not have to be in the longitudinal center of the vehicle, and it may have more than two driven axles. The steering lock is referred to in the following text as the steering value, and is either the steering angle of a vehicle with steering-knuckle steering or the articulation angle of a vehicle with articulated-vehicle steering.
Although it is know, from EP 605 559 B1, to brake individual wheels in a controllable manner in the case of vehicles having articulated-vehicle steering, in order to improve traction, and in order to avoid the problems when using a lockable differential, in particular when driving through curves, in this case the wheel rotational speed differential signals are, however, corrected only as a function of the articulation angle (=steering value).
The use of friction brakes (as distinct from friction clutches between the elements of the differential) is a disadvantage, however, because of high energy losses and a reduction in the total torque that is available. The correction of the wheel rotational speed differential signals does not lead to the target for the locking control system, since it cannot be detected whether the pulses additionally caused by the steering movement have to be added with a positive or negative sign. It is also not possible to detect whether the traction loss (as a result of spinning of a wheel or several wheels) relates to the slower or faster output shaft, induced by the kinematic slip. The kinematic slip is to be understood as the rotational speed differences between the wheels as a result of the different track radii of the individual wheels in the curve. In addition, the influence of the steering value change in this known vehicle remains completely unconsidered, which can lead to the vehicle becoming unsteerable as the result of brake engagement on one side (only), or the steering becomes overloaded and breaks.